It’s almost too simple for words. It’s so easily missed because its discovery does little to gratify the inventor’s creative instinct, or the engineer’s hard-won high-tech education, or the tinkerer’s love of gadgetry. New inventions, theories, and gizmos are so unnecessary as to distract from compressed air’s ultimate secret, which is really just efficiency, inherent and designed-in.

Compressed air's inherent efficiency is made evident by these facts:

_Air is everywhere.
_Air contains solar energy.
_Compressing air is a simple process that makes its internal energy (solar heat) usable without altering the air chemically. No thermodynamic conversions or changes of state are necessary, eliminating wasteful steps.
_The energy wasted in compressing air takes the form of heat, which is air engine fuel, and is conservable for use.

Design efficiency is the need to go one step further than status quo convention in the production and use of compressed air. The two-step process of compressing and expanding air creates two opportunities to introduce efficient design measures, that is, to conserve energy.

_Means of expanding air can be provided to use more of the available energy before exhausting it, by way of a relatively efficient air engine as opposed to a commercial air motor.
_Means of conserving compressor work can be used to decrease the net cost of making compressed air.

The ultimate secret of compressed air is that to create a self-fueling pneumatic power plant, all you have to do is make the most of the energy contained in expanding air, and/or make the most of the energy invested in compressing air. Simply put, the self-fueling air engine is a natural phenomenon of ordinary processes, and not some aberration of fringe science or a result of exotic devices. This basic fact is easily proved mathematically using standard engineering formulas and charts. Gizmos and gadgets are fun, but unnecessary; they are the stuff of research institutes. The first goal should be to show an ordinary air engine running an ordinary compressor and keeping its own tank full in the process. I repeat: the math easily proves this possible.

The idealization of conserving compressor work would be to put the compressor in the tank. The fresh air brought in from outside the tank is compressed into the tank, and the work done in the compressing dissipates into the tank as heat, expanding the volume of air available to the engine as fuel. The increase in fuel value due to the conserving of compression heat represents a value beyond what one would expect from engineering charts but not beyond the scope of ordinary engineering formulas to quantify.

The idealization of conserving expansion potential is to expand the compressed air so slowly that its pressure never goes down, since the heat used to push pistons is replaced by ambient heat absorbed from the surroundings. This is straight out of the thermodynamics textbook.

The obvious impracticalities of these idealizations are beside the point; it remains only to identify means of going in their general direction. For example:

_An insulated shroud enclosing both the compressor head and the engine head would conserve much of the compression heat.
_A multi-stage engine with inter-stage ambient heaters, which runs at a low RPM, would squeeze lots of work out of a little air.
_Combining the two strategies could lead to the most practical design.

Gizmos can be added later—such as reducing compression work (as opposed to conserving the work of a normal compressor)—with a series of check valves (and/or jet pump in the tank) that allows low pressure air to be injected into a high pressure tank. Other possibilities include:

_heat pipes
_electric resistance heaters
_water-cooled compressors that dump heat into channels in the air engine block
_As a last resort, any air engine can be made more efficient by using combustible fuel to heat engine air till the Coefficient of Performance (COP—see heat pump basics) rises above unity, making a hybrid power plant without the noise, pollution, and expense of an internal combustion engine.

If not for compressed air’s simplicity, its use in solar power production would have been mastered long ago. We must stop flattering ourselves with our brilliant new ideas, and prove the self-fueling nature of air with basic designs that take advantage of air’s best feature: its ultimate simplicity.

FROM FALSE ANALOGIES TO FREEDOM FROM FUEL
To state that compressing air gives it the ability to do work is like saying that building a dam gives water the ability to operate a power-producing turbine. While these are true statements, they are not made in the scientific context of energy investments. It would be false to state that the work invested in compressing air results directly in the work compressed air can do, just as it would obviously be false to credit the work of building a dam for the quantity of water power thus made available. In both cases, the work done by the pressurized fluid is a result, scientifically speaking, of the sun’s energy, while the work of the compressor, like that of the dam builder, is an incidental investment—you might say an economical consideration or hardware cost—rather than a scientifically correct accounting of the energy invested that later pays off in the power made available.

WHAT’S RIGHT WITH WHAT’S WRONG WITH AIR
Some of air’s so-called disadvantages, according to the usual way of looking at things—a viewpoint that is built around using air for safety, portability, and convenience, not for efficiency—are some of its greatest advantages.
The classic case of this glaring discrepancy between standard thinking and air’s real potential is the assumption that air is self-defeating in any attempt to produce power because of the fact that it gets cold. The standard line goes like this: air enters a cylinder of an air motor and expands, becoming cold in the process. The air subsequently entering the cylinder will be cooled by the cylinder walls, robbing part of its power value before it can do any work. Effectively then, it is a self-defeating hope to try and use air efficiently. My rebuttal is that the cold produced by compressed air’s expansion in a cylinder is an advantage because it makes the machinery a sponge for heat in the surrounding atmosphere which, if absorbed into the system because of enlightened design work, becomes free energy for the piston to use.
Let’s face it: once Americans get even the vaguest inkling that anyone or anything could be considered wimpy, they shun it like the plague. Could it be that the general ignorance of compressed air’s subtle and misunderstood nature is a result of our macho nature, our fear of being associated with sissified ideas?
Once I called the manufacturer of a fairly efficient compressed air motor, and when I asked why the motor wasn’t being put in cars, the salesman I spoke to informed me that his boss would remind me that in order for the car to go anywhere, it would have to be followed by a semi truck carrying its compressed air supply.
Now if that isn’t self-defeating and wimpy, I don’t know what is. It’s downright Unamerican to give up so easily!
Similarly, it is thought to be ever-so ridiculous, that a compressor on-board an air car would be the silliest notion since self-chewing bubble gum. All that power wasted, for the little trickle of compressed air made available.
But wait a minute. In what way is all that power used?
It is used to make heat.
And what is it about compressed air that makes it capable of pushing pistons?
It’s the heat.
And what is it about expanding air that makes it seem so objectionable as a piston-pushing medium?
The cold produced.
And what does cold do to heat?
It sucks it up like a sponge.
Conclusion: the hotter air gets when it's compressed, the more heat is available to be conserved; the colder the air gets when it's expanded, the more ambient heat it can absorb from outside the engine.